This invention relates to an automatic light intensity control system for a high-density X-ray film viewer.
Industrial X-ray pictures are employed for inspecting the interior of a metallic member or casting, or a weld of such a casting, and, in general, such pictures have a very wide density range. Accordingly, in order to enable the detection of defects from a high-density film, a viewer capable of providing a high degree of illumination must be used. With such a viewer, however, an observer's eyes are quickly dazzled when directly exposed to the bare viewer screen, and he therefore becomes easily fatigued. Even when a film is placed in the viewer, depending upon the configuration and the density of the photographed object, detailed observation is difficult and tedious because the brightness is excessive or the contrast is too low. To overcome this difficulty, a viewing device with a manual light intensity control has been proposed. However, such a device still suffers from a variety of disadvantages, as described below. Whenever a film is placed in the viewer, the intensity of the light must be manually adjusted, and it is therefore difficult to maintain constant illumination conditions at all times. Further, when the film is removed the observer's eyes are exposed to intense light and are therefore dazzled or temporarily blinded. To avoid this the light control must be manually adjusted whenever a film is inserted in or removed from the viewer.